This invention relates to long-lasting stable aqueous chlorine dioxide solutions, as well as methods of making and using said solutions. Aqueous chlorine dioxide solutions that are long-lasting and stable are particularly useful in applications requiring relatively small amounts of chlorine dioxide treatment or intermittent treatments. Chlorine dioxide is a strong oxidizing agent finding application in a wide range of fields including bleaching, pesticides, biocides, pollution abatement, disinfectant, oil field uses, and the like.
Chlorine dioxide is commercially employed as a bleaching, fumigating, sanitizing, or sterilizing agent. Chlorine dioxide can be used to replace chlorine and hypochlorite products more particularly used in such applications with resultant benefits. Chlorine dioxide is a more powerful sterilizing agent and requires lower dose levels than chlorine. Chlorine dioxide produces lower levels of chlorinated organic compounds than chlorine when it is used to sterilize raw water containing organic compounds. Additionally, chlorine dioxide is less corrosive than chlorine to metals.
However, chlorine dioxide is an unstable, highly reactive gas which is soluble in and decomposes in water. See, e.g., U.S. Pat. No. 4,941,917. Therefore, it has heretofore been necessary to generate aqueous chlorine dioxide solutions on site for immediate use or use within a relatively short time (typically less than an hour). Due to its poor stability, it has been the practice in the industry to store chlorine dioxide in the absence of light and at reduced temperatures. This requirement of on site generation of chlorine dioxide has severely limited its utility in facilities requiring relatively small amounts of chlorine dioxide or those which have only intermittent needs.
There are a number of known methods for producing chlorine dioxide. For large chlorine dioxide requirements, it is typical to employ a chlorine dioxide generator. See for example U.S. Pat. Nos. 4,247,531, 4,590,057, and 5,204,081.
The “hypochlorous acid process,” commonly achieved through depression of pH on chlorine dioxide generators, which includes many commercially available chlorite based chlorine dioxide systems, are actually not chlorine gas sodium chlorite reactions. These systems depend on the reaction of chlorine dissolved in water in the form of hypochlorus acid with sodium chlorite to form chlorine dioxide. This reaction is described in Equation 1:HOCl+HCl+2ClO2−→2ClO2+H2O+2Cl−  Equation 1
For hypochlorite ion to be predominantly in the form of hypochlorus acid the pH of the reactant solution must be below 2.8. Otherwise the dissolved chlorine will be in the form of hypochlorite ion. If this is the case the predominant reaction that occurs is shown by Equation 2:OCl−+ClO2−→ClO3−+Cl−  Equation 2
Thus at pH values above 2.8, the predominant end product of these type chlorine dioxide generators is chlorate and not chlorine dioxide.
The pH can be too low for these types of chlorine dioxide generators. At pH values below 2.3 another side reaction starts to take place as described in Equation 3:ClO2−+4H+3Cl−→2Cl2+2H2O  Equation 3
If pH is driven too low in these systems chlorite can decompose to chlorine. Therefore in hypochlorus acid type systems the operating pH range should be maintained between 2.3 and 2.8. Under laboratory conditions it has been found that in this pH, and at a concentration of 1500 mg/l of reactant chlorite, 92% of the chlorite will be converted to chlorine dioxide (Equation 1), 4% will be converted to chlorate (Equation 2), and 4% will be converted to chlorine (Equation 3). As these reactions all proceed at about the same rate, the actual pH governs the actual percentages formed.
In some applications it is particularly desirable to have chlorine free solutions of chlorine dioxide. This can be achieved with the above solutions by complex purification steps involving stripping the chlorine from solution and re-absorption of chlorine dioxide from a generating solution to a receiving solution. A stream of air is typically used for this purpose. But, this is hazardous if the concentration of chlorine dioxide in the air stream becomes high enough to initiate spontaneous decomposition. See, U.S. Pat. No. 6,274,009. Concentrations of chlorine dioxide in air above 11% can be mildly explosive.
Chlorine gas reacts rapidly with solid or aqueous sodium chlorite. In one type of chlorine dioxide generator, precursor chemicals are pre-reacted, under vacuum, in the absence of dilution water. See, U.S. Pat. No. 6,645,457. The reaction scheme for this chlorine gas sodium chlorite reaction is as follows:2NaClO2+Cl2→2ClO2+2NaCl  Equation 4.
It is an advantage in this type of system that it does not dissolve chlorine gas in the dilution water. Rather, the chlorine gas is pre-reacted to form chlorine dioxide gas which is then dissolved in the dilution water.
In addition to high capital costs, these systems also require an experienced chemical operator. So, this type of system is not practical for users who require only small quantities of chlorine dioxide or require it only intermittently. Additionally, for many small or intermittent treatments, the functionality of small generation systems may not be reliable. The small application cannot justify the technical support and monitoring required to ensure the quality control parameters necessary in such operations.
As an alternative to the above generator technologies, some small scale chlorine dioxide users have turned to the so called “stabilized chlorine dioxide” products. These products are aqueous solutions of sodium chlorite that are activated at the site of use with a weak acid to produce chlorine dioxide in solution. Examples of stabilized chlorine dioxide products are found in U.S. Pat. Nos. 4,964,466, 4,941,917, and 5,031,700. A significant disadvantage of stabilized chlorine dioxide is that the weak acid activation of the chlorite is highly inefficient, producing typically less than 20% yield stabilized chlorine dioxide.
Summarizing the state of chlorine dioxide treatments, there is currently no efficient method for many industrial applications requiring relatively small amounts of chlorine dioxide.